Forward Gear Bearing – Fits Mercury/Mariner Outboards (Replaces 31-78172A1)

$45.66 $73.00

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Forward Gear Bearing – Fits Mercury/Mariner Outboards (Replaces 31-78172A1) is backordered and will ship as soon as it is back in stock.

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Product Details

This is a Forward Gear Bearing – Fits Mercury/Mariner Outboards (Replaces 31-78172A1), designed as a direct-fit tapered roller bearing for the forward gear assembly. It is compatible with a range of Mercury and Mariner outboard engines from 105 to 225 HP that utilize the specified OEM part number.

This component provides a direct interchange for Mercury 31-78172A1, GLM 21830, EMP 31-02131, and Red Rhino R-205149/205110.

Installs in the forward gear assembly of the lower unit.
Constructed from high-strength steel for durability.
Includes one complete bearing assembly for a full replacement.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
Mercury / Mariner	31-78172A1
GLM	21830
EMP	31-02131
Red Rhino	R-205149/205110

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury/Mariner models, verify this part is compatible with your specific 105-225 HP outboard by checking against the OEM part number 31-78172A1.

Brand	Model	Rated HP	OEM Part No.	Notes
Mercury/Mariner	Various Outboards	105-225	31-78172A1	Forward gear bearing

Technical Specs

Product Type: Tapered Roller Bearing
Installation: Forward Gear Assembly
Housing Material: Steel
Product Dimensions: Length: 1.31 in, Width: 3.44 in, Height: 3.5 in
Weight: 1.63 lb (0.74 kg)
OEM Part Number: 31-78172A1

What’s in the Box

1 x Bearing Assembly (Replaces 31-78172A1)

Installation Notes

Drain the gearcase oil completely.
Disassemble the lower unit to access the forward gear and bearing assembly.
Carefully remove the old bearing, taking note of any shims or washers and their orientation.
Clean the bearing carrier and surrounding area to remove any debris or old lubricant.
Press the new bearing evenly into place using an appropriate bearing driver or press.
Reassemble the gearcase components, ensuring all shims are correctly positioned and fasteners are torqued to manufacturer specifications.

Troubleshooting & FAQ

How do I confirm this bearing fits my specific outboard model?

The best way is to match the OEM part number. This bearing replaces Mercury part 31-78172A1. If your engine's service manual or parts diagram specifies this number for the forward gear bearing, it will fit.

What are common signs of a failing forward gear bearing?

Common symptoms include a grinding or whining noise from the lower unit (especially in forward gear), excessive metal shavings on the gearcase drain plug magnet, or difficulty shifting into forward gear.

What is the recommended replacement interval for this bearing?

There is no fixed replacement interval. The bearing should be inspected for signs of wear, pitting, or roughness whenever the lower unit is serviced for other reasons (e.g., water pump replacement) and replaced if any damage is found.

Is this bearing pre-greased?

No. The bearing should be lubricated with fresh marine gearcase oil during installation. It operates within the sealed gearcase and is lubricated by the gear oil.

Could a bad bearing cause other damage?

Yes. A failed forward gear bearing can lead to catastrophic damage to the forward gear, pinion gear, and other internal components of the lower unit if not addressed promptly.

OEM vs. Aftermarket?

This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials and manufactured in an ISO 9002 facility, where it is batch-inspected for consistency.

Shipping speed & cost?

We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options appear at checkout where available.

Warranty & returns?

This product is covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window for unused items in original condition. See our policy page or reach out for instructions.

Safety & Important Info

Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
This is an aftermarket replacement, not an OEM component.
Professional installation is recommended for safety and performance.

About Sea Sierra

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Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.

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